(1) Field of the Invention
The present invention relates to an optical low pass filter to be used in an optical system for a single or double tube color television camera including a color encoding filter for obtaining chrominance signals.
(2) Description of the Prior Art
There have been proposed and known various types of optical low pass filters in the prior art which are used in the single or double tube color television camera for eliminating cross-talk between the luminance and the chrominance signals by separating the spancial frequency band in which these signals lie. One example of the filters is a rectangular-wave phase grating filter which can provide a signal low pass effect without a substantial loss of light during the transmission, see for example, U.S. Pat. No. 3,681,519, U.S. Pat. No. 3,756,695 and U.S. Pat. No. 3,821,795. This type of low pass filter can provide OTF (Optical Transfer Function) characteristics wherein the OTF value is reduced over a substantial range of the spatial frequency as shown by line A in the attached FIG. 1. However, it has a disadvantage that the OTF value linearly decreases with the spatial frequency, resulting in low OTF values at lower spatial frequency range i.e. the band of the frequency higher than the cut-off frequency (also see line A in FIG. 1), consequently the image of an object obtained through the filter has not desirable contrast and sharpness.
The prior art has also used double refraction or birefringent plates such as shown in U.S. Pat. No. 3,588,224 which discloses application of the birefringent plate as an optical low pass filter. This type of optical low pass filter has comparatively high OTF values at the lower spatial frequency range. However, the OTF characteristics obtained through the birefringent low pass filter is such as shown by the broken line B in FIG. 1. That is, the OTF value decreases at a specific spatial frequency but not over a desirable wide range of the spatial frequency.
Accordingly, the primary object of the present invention is to provide an improved optical low pass filter having the joint advantages of both the rectangular-wave phase grating type and the birefringent type filters. Another object of the present invention is to provide an optical low pass filter having an OTF characteristic wherein the OTF value is substantially high at the lower frequency range i.e. in the band shared for the luminance signals and is reduced or cut-off over a desirably wide range in the band of the spatial frequency higher than the cut-off frequency, i.e. the band to be shared for the chrominance signals. To accomplish these objects, the present invention adopts a triangular-wave phase grating filter.
To use a triangular-wave phase grating as the optical low pass filter for a single or double tube color television camera, has been proposed in U.S. Pat. No. 2,733,291 and aforesaid U.S. Pat. No. 3,821,795. However, as such prior proposals directed an attention to or suggested utilization of only the refraction effect at the inclined surfaces of the triangular-wave grating, it has been believed that the OTF characteristics obtained are substantially the same as is obtained by the birefringent filter, i.e. as shown by B in FIG. 1, and that the triangular-wave phase grating can not provide reduction of the OTF value over a wide range. In addition, manufacturing of the triangular-wave phase grating filters of expected configulations has been difficult. Thus, the triangular-wave phase grating optical low pass filter has not been brought into practice.